Magnetoresistive Random Access Memory (MRAM) is a non-volatile memory technology that uses magnetic elements. For example, Spin Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM) uses electrons that become spin-polarized as the electrons pass through a thin film (spin filter). STT-MRAM is also known as Spin Transfer Torque RAM (STT-RAM), Spin Torque Transfer Magnetization Switching RAM (Spin-RAM), and Spin Momentum Transfer (SMT-RAM).
Referring to FIG. 1, a diagram of a conventional STT-MRAM cell 100 is illustrated. The STT-MRAM bit cell 100 includes a magnetic tunnel junction (MTJ) storage element 105, transistor 110, bit line 120, and word line 130. The MTJ storage element 105 is formed, for example, from a pinned layer and a free layer, each of which can hold a magnetic field, separated by an insulating (tunnel barrier) layer as illustrated in FIG. 1. The STT-MRAM bit cell 100 also includes a source line 140, sense amplifier 150, read/write circuitry 160 and bit line reference 170. The source line 140 is connected via a pad (not shown) to the lower portion 114 of the transistor 110.
Those skilled in the art will appreciate that the magnetic tunnel junction (MTJ) storage element 105 is grown on an additional metal layer known as a bottom electrode (BE) plate 180, which is connected via a seed (not shown) to the upper portion 112 of the transistor 110. The mechanical surface properties of the BE plate 180, such as the surface flatness or roughness, affect the performance of the MTJ storage element 105. The BE plate 180 generally is formed from a rigid, polished metal, such as a titanium alloy or like metal having mechanical properties suitable for forming the MTJ storage element 105 thereon.
With reference to FIGS. 1 and 2, a conventional STT-MRAM cell 100 has a magnetic tunnel junction (MTJ) storage element 105 formed on a hexagonal (or octagonal) bottom electrode (BE) plate 180. The STT-MRAM cell 100 also has a seed 190 that connects the BE plate 180 to the upper portion 112 of the transistor 110.
As shown in FIG. 2, in conventional, non-symmetric STT-MRAM cell array designs, the corners of the bottom electrode (BE) plate 180 are removed or etched off to form the hexagonal (or octagonal) shape in order to maintain reduce spacing requirements between adjacent BE plates in the conventional STT-MRAM cell array. The hexagonal bottom electrode (BE) plate 180 has a width X0 and a height Y0. In conventional STT-MRAM cells, the BE plate width X0, active layer width, and metal widths are substantially different, as can be seen in FIG. 2.
Those skilled in the art will appreciate the operation and construction of the memory cell 100 is known in the art. Additional details are provided, for example, in M. Hosomi, et al., A Novel Nonvolatile Memory with Spin Transfer Torque Magnetoresistive Magnetization Switching: Spin-RAM, proceedings of IEDM conference (2005), which is incorporated herein by reference in its entirety.